EP0249659A2 - Screw spindle, screw device and process for their use - Google Patents

Abstract

A power-wrench bolting spindle (1) is used to insert and tighten screws or nuts. The bolting spindle is driven at one end by a motor through a reduction gear (5) and at its other end a bolting tool is seated. A torque sensor (7) is mounted between the motor and the spindle. During bolting, other essential data besides the bolting torque are ascertained. The bolting spindle is equipped with a depth sensor (24) connected to the regulating circuit to measure the depth of bolting. An angular-speed sensor is connected to the regulating circuit and measures the motor or spindle angular speed. The bolting procedure is carried out in two operational stages separated by a pause, namely an application stage (L) and a tightening stage (Z). Each of these operational stages terminates when a specific shutdown bolting torque is reached. Due to the control by the depth sensor, the bolting angular speed is lowered from its initial value when approaching the application stage to a final value which, when the shutdown bolting torque is reached, allows stopping without overloading.

Description

The invention first relates to a screw with the components mentioned in the preamble of claim 1.

Screw spindles of this type are used in machine production, e.g. used in the automotive industry. Application examples are screwing an engine block onto the load-bearing vehicle parts, screwing on the cylinder head cover, screwing together the components of connecting rod bearings.

In order to effectively control a screwing process, it is desirable to continuously measure the screwing depth. With the help of a depth sensor, it is possible to lower the speed in the application stage just before the screw head or nut comes into contact with the base, so that the application stage can be ended with high accuracy when the specified application torque is reached. An accuracy of 1% of the torque should be achievable.

The depth measurement is used for a further check: If the hole is too short for a screw or if the thread does not reach far enough into the hole (both referred to as a "blind hole") or if there is a foreign body in the hole, the depth check and simultaneous Torque control determined that the tightening torque had not yet reached the required screw depth, and an error signal can be given.

From DE-OS 29 30 430 a screw spindle of the type mentioned is known. It is equipped with a magnetic sensor (54) for the angular position of the output shaft (26). A screw is screwed in until its head comes into contact with the surface of its Workpiece comes. In a second phase, the screw is tightened to a predetermined torque under the supervision of the angular position sensor.

A depth measurement is not possible directly with the well-known screw spindle. Rather, the screw depth must be calculated from the number of revolutions, taking into account the thread pitch. However, this makes it necessary to specify parameters for the thread pitches.

The present invention is intended to create a screw spindle, during the operation of which the screw-in depth itself can be measured continuously.

This object is achieved according to claim 1.

The spindle is then equipped with a depth sensor that generates signals during the screwing process about the screwing depth achieved and / or the remaining distance between screwing head or nut and the base, which can be given to a control circuit. The absolute screw-in depth is measured regardless of the number of revolutions of the spindle, the thread pitch and the screw-in time.

Developments of the invention

A development of the invention according to claim 2 relates to a screwing machine. In this, when using a screw spindle according to claim 1, additional length meters can be provided, which determine whether a workpiece that has approached, for example an engine block, is in a position within permissible tolerances with respect to a mounting plate holding the screw spindle, or is crooked or is too small or is too far apart.

In production lines in the automotive industry, engines with cylinder blocks and / or cylinder heads made of steel and aluminum must be processed one after the other, depending on the delivery program, for which the same screwing machines and spindles must be used. Depending on the material, the bolts must be tightened with different tightening torques. The operator is therefore required to convert the screwing machines to the required torques. In addition to the torque, other data such as the gear ratio of the motor reduction gear, the maximum motor speed and the angular resolution must also be entered.

Among other things, the engine speed is essential for the screwing of steel or aluminum, since different friction values occur with both materials. Steel has a much rougher surface, so that lower speeds have to be used for steel than for aluminum, so that there is no heat development and the cold welding mentioned above. On the other hand, in order to save time, aluminum should be screwed at the higher permissible speed.

The data required for a particular screw former is contained in data sheets. The correct data sheets must be selected and the data entered by hand, which, apart from the amount of work involved, entails the risk of incorrect entries and therefore poorer (too loose or too tight and welded) screw connections.

In addition, externally identical screw spindles can have different reduction gears without this being always clearly stated on the screw spindles.

In a further development of the invention according to claim 3, the screwing torque and the rotational speed are also to be recorded using a screw spindle according to claim 1. The three corresponding measurement data are to be used in the manner of a real regulation for controlling the screwing process, so that on the one hand the screwing process is very difficult less time can be carried out, but on the other hand, an unduly high friction between the parts to be screwed and thus welding ("cold welding") can be avoided.

Taking into account these continuously available measurement variables, it is possible to optimally carry out the screwing process, the screw or nut being tightened to the desired tightening torque, and the screwing being carried out in the shortest possible time and without damage.

The screwing process is carried out in two stages, namely first a lay-on stage, which ends when the screwing head or nut touches its base. After a dwell time, a second stage begins, the tightening stage, in which the screw connection is tightened to a specified torque.

Different screw spindles must be used for different screwing processes (different materials of the parts to be screwed as well as different lengths and thicknesses of the screws). Each screw spindle must be connected to the control circuit after it has been installed, and the control circuit must be informed in some way of the parameters of the screw spindle and its drive. So that this does not have to be done by hand, which takes time and is a source of error, an electronic memory module can be provided according to claim 3, which is accommodated in the screw spindle itself and which, when connected to the control circuit, supplies it with the necessary data for its setting.

According to claim 4 can be achieved that the electronic memory module, which contains the parameters of the screw spindle, its drive and the sensors, when connecting a screw spindle to the control circuit on its own, via the torque sensor of the screw spindle, emits a pulse train to the control circuit , which feeds all the necessary parameters into the control circuit, making this the screw spindle is adjusted.

The invention also relates to a method for operating the screwing machine. Thus, according to claim 5, the screwing process can be carried out in two process stages separated by a dwell time, namely the lay-on stage and the tightening stage, each of these stages being able to be ended when a predetermined switch-off value, namely a torque or a motor current, is reached. In this case, the screw speed can be reduced to a permissible final value when approaching the application state in the application stage under depth measurement, which allows switching off without endangering the screwed parts.

According to claim 6, the screw speed n can be controlled in both stages under the influence of the respectively measured screw torque M according to a very simple formula, according to which the speed is reduced with increasing screw torque. This will definitely exclude the damage described above when putting on or tightening.

According to claim 7, the motor current actual value I can be used as the control criterion instead of the screwing torque.

• Figur 1 zeigt in Seitenansicht eine Schraubspindel nach der Erfindung.
• Figur 2 zeigt eine Schaltung mit einem Speicherbaustein und dient zum Einspeichern von Einstelldaten indie Regelschaltung.
• Figur 3 zeigt den zeitlichen Verlauf eines Clock-Eingangssignals für den Speicherbaustein.
• Figur 4 zeigt den Verlauf eines typischen Ausgangssignals des Spei­cherbausteins.
• Figur 5 zeigt den zeitlichen Verlauf des Schraubdrehmoments M in der Anlege- und der Anziehstufe.
• Figur 6 zeigt entsprechend den Verlauf der Motordrehzahl n.

Exemplary embodiments with further features of the invention are described below with reference to the drawings.

• Figure 1 shows a side view of a screw according to the invention.
• Figure 2 shows a circuit with a memory module and is used to store setting data in the control circuit.
• FIG. 3 shows the time course of a clock input signal for the memory chip.
• FIG. 4 shows the course of a typical output signal of the memory chip.
• FIG. 5 shows the time profile of the screwing torque M in the application and tightening stages.
• Figure 6 shows the course of the engine speed n.

Figure 1 shows an embodiment of a screw screw 1, which is fixed in a known manner on a mounting plate 2 and with this perpendicular to a workpiece 3, z. B. the cylinder head cover of an engine block can be moved vertically.

Between the mounting plate 2 and the workpiece 3, at least one (here only shown schematically) commercial length meter 4 is arranged, which functions as a depth meter and determines the distance between the mounting plate 2 and the surface of the workpiece 3. The depth gauge can work purely mechanically or with waves, e.g. B. radar or ultrasound. Preferably, three such length meters are arranged in the ring around the axis of the screw spindle 1, namely at mutual angular intervals of 120 °. As a result, not only the distance between the mounting plate and the workpiece can be determined, but also any misalignment of the workpiece. This information are essential for checking the current depth measurement by the screw spindle 1 itself.

The screw spindle 1 is driven by a brushless motor, not shown here, via a reduction gear 5, which is only shown schematically and partially. The reduction gear has a constant reduction ratio, so it cannot be switched to a different output speed. A specific motor and a specific reduction gear are provided for each screw spindle. Both are chosen especially for the diameter of the screw or nut. The output shaft 6 of the reduction gear is non-rotatably connected to the input of a torque sensor 7, which is usually also referred to as a "load cell" or "load cell flange". The torque sensor is used to measure the torque transmitted from the motor to the screw spindle and screw or nut during a screwing process.

A flange 8 is used to attach the screw to the mounting plate 2, which, for. B. can be brought into a position of use by a swivel arm. The flange 8 carries a bearing housing 9 in which the drive shaft 11 of the screw spindle is rotatably mounted. The drive shaft has 8 axially parallel grooves and ribs below the flange and supports a tubular output shaft 13 with similar grooves and ribs so as to be displaceable in the axial direction. The end receiving the torque, the receiving end 13.1 of the output shaft 13 is thus inserted in the manner of a multi-spline hub over the end giving off the torque, the output end 11.1 of the drive shaft 11. The output shaft is closed at the bottom and carries a square 15 or the like. For attaching a screwing tool, in particular a wrench, which is then placed on the screw head or the nut. (Wrench and screw head or nut were not shown.)

A protective sleeve 17 projects from the flange 8 downward in FIG. 1 and is screwed onto a threaded connection of the flange 8, that is to say it can be removed. The protective sleeve contains a coil 20. The hollow output shaft 13, which consists of ferromagnetic material, in particular tool steel, represents a coil core. The coil 20 is surrounded by a helical spring 22, which strives to push the output shaft out of the protective sleeve 17 (up to an end stop, not shown). In this way, a depth sensor is formed, which is denoted here as a whole by 24. In the position shown, the output shaft 13 plunges approximately half into the coil 20, which corresponds to a specific inductance of the coil. The inductance of the coil is changed by changing the indentation depth (i.e. pushing up or pushing down the output shaft 13).

Lines, not shown in FIG. 1, continuously supply the resulting measurement data, namely inductance of the coil as parameters for a screw depth and the torque, to a control circuit, not shown.

FIG. 2 shows, in a basic circuit, a bridge 26 which is constructed from four strain gauges 28a to 28d. The strain gauges are arranged in the torque sensor 7 in a manner known per se in such a way that their effects mutually increase, in that two tensile and two oppositely connected compressive stresses are used. Instead of four, only two strain gauges and two fixed resistors can be used in a manner known per se. Points 32 and 33 of the bridge are connected to a DC supply voltage. Points 30 and 31 represent the bridge output, from which a control signal is sent to the control circuit, not shown. As far as described so far, this circuit is used to transmit the respective torque to the control circuit.

In addition, this circuit should serve to transfer parameters that are necessary for the basic setting of the control circuit, suitable for the respective screw spindle and its drive.

All required parameters are stored in a memory module 35, in particular a semiconductor module. The memory chip is a programmable and readable semiconductor memory. The information contained in it is read out in serial data transmission.

A resistor 38 is connected in parallel to the strain gauge 28a via an electronic switch 37 (here an NPN transistor). The base of the transistor is connected to an output 40 of the memory chip 35. The memory chip has a clock input 42.

FIG. 3 shows a chronological sequence of clock signals which are fed to the clock input 42. Corresponding to the data stored in the memory module 35, a pulse sequence results at its output 40, an example of which is shown in FIG. Depending on the signals occurring at the output 40 and thus at the collector of the transistor 37, the resistor 38 is alternately connected in parallel to the strain gauge 28a or the parallel connection is interrupted. The bridge is therefore detuned in rhythm with the output signals of the memory chip. The torque signal to be emitted by the bridge is thus superimposed on the clock signals by a sequence of signals which contain information about the essential parameters.

The following parameters can be stored in the memory module 35 for a specific screw spindle, which is equipped with a specific torque sensor and is provided with a specific motor and reduction gear:
Motor: Maximum speed and
Maximum power consumption
Gearbox: gearbox ratio
Torque sensor (load cell): nominal size in Nm
Sensitivity in mV / V
Depth sensor: depth in mm / V
Overall efficiency of the screw spindle: ratio of the supplied to the output power

In the control circuit, the following values are calculated from the values read in from the memory module, which will later be used to control the tightening processes:
Maximum speed of the output shaft as a quotient of the maximum engine speed and the gear factor and the gear reduction;
Angular resolution as impulses / degree of commutation of the brushless motor, multiplied by the gear reduction;
The efficiency W.

The commutation pulses are picked up directly by the motor. You can e.g. B. have a resolution of 7.5 ° / pulse.

The efficiency W must be monitored during a screwdriving process, namely the quotient of the power removed and the power supplied
W = P _from : P _to (1)

Here P _zu = U · I, namely the product of the current drawn by the motor at its input voltage.

The power P _ab removed is a function f of the respective screwing torque and the respective screwing speed
P _ab = f (M · n) (2)

If the efficiency W becomes less than the predetermined efficiency calculated according to Formula 1 during a screwing process, this is an indication of increased friction within the screw spindle bearings and announces a possible failure of the screw spindle. It will be a warning sign educated.

A signal sequence that contains these parameters is only transferred to the control circuit at the beginning, namely after the screw spindle has been installed and upon request of the control circuit. The parameters then stored in the control circuit serve as the basis for the control in all similar screwing operations of the same screw spindle.

If the screw spindle is replaced and / or the data of the screwing process is changed, such as the material, length and diameter of the screw, additional data must be entered for this, but it is not in the memory module. The memory module is located in the screw spindle itself and only contains the parameters typical for the screw spindle and its drive.

FIGS. 5 and 6 show the time profile of the screwing torque M or the screwing speed n in a typical screwing process. As mentioned, the screwing into two time segments is carried out, the on egestufe l L and the at z iehstufe Z, which are separated by a dwell V.

The landing stage begins with the maximum speed n _max and an initially increasing screw torque M. While z. B. the screw is screwed in, moves (Figure 1) under the influence of the coil spring 22, the output shaft 13 relative to the protective sleeve 17 down, whereby the inductance of the coil decreases. The signal given by the coil is converted into a measure of the screw depth and compared with a previously set final value of the screw depth. The final value corresponds to the contact of the screw head or the nut on the base. Under the influence of the depth signal, the tightening torque M is reduced so that it reaches a finite value M _AL at the end of the application stage L.

worin bedeuten:
n_max maximale Schraubdrehzahl in der Anlege- bzw. Anziehstufe
n_min minimale Schraubdrehzahl in der Anlege- bzw. Anziehstufe
M_A Abschalt-Schraubdrehmoment am Ende der Anlege- bzw. AnziehstufeThe speed n is inevitably reduced, according to the formula

in which mean:
n _max maximum screw speed in the application or tightening stage
n _min minimum screw speed in the application or tightening stage
M _A switch-off tightening torque at the end of the application or tightening stage

Formula 3 simply means:
n = n _max - km (4)
where k is a constant.

The speed n is therefore reduced from its maximum value, and it becomes smaller with increasing torque until it reaches a final value that is greater than zero.

Formula 1 applies to both the application level L and the tightening stage Z. The value M _A means the tightening torque that is to be achieved at the end of the application or tightening stage.

The landing stage is followed by a dwell time, which is intended to ensure that the landing stage is completed in all cases when several screws or the like are to be screwed in. If a screwing process is not feasible, e.g. B. because a blind hole is present, the entire screwing process can now be interrupted or, after eliminating a fault, for example removing a foreign body from the screw hole, for which a screw is repeated and then continued for everyone in the tightening step.

During a tightening process, the tightening torque is continuously determined and read into the control circuit. A shut-off torque M _{A is} specified for both the landing stage and the tightening stage. These shut-off screw torques have a low value for the contact step and a significantly higher value for the tightening step. The control circuit regulates the associated screw speed for each continuously measured and read screw torque according to Formula 3.

In the landing stage, as shown in FIG. 6 and as can be seen from formula 3, the speed is not reduced to zero but to a safe value n _AL , which cannot lead to mechanical damage, but allows the screwing process to be carried out in an optimally short manner Time to perform.

In the tightening stage, the maximum speed n _{max is} started again. The speed is again regulated using Formula 3, here to a final value n _AZ . The tightening torque is increased to a predetermined final value M _AZ , which results from formula 3.

In both stages, the motor is switched off when the specified shut-off screw torque M _AL or M _{AZ is reached} .

worin bedeuten:
n_max maximale Schraubdrehzahl in der Anlege- bzw. Anziehstufe
n_min minimale Schraubdrehzahl in der Anlege- bzw. Anziehstufe
I_A Abschalt-Motorstrom am Ende der Anlege- bzw. AnziehstufeThe actual motor current I can be used as a criterion for the regulation of the screw speed instead of the screw torque. This then results in the following formula, which is used to regulate:

in which mean:
n _max maximum screw speed in the application or tightening stage
n _min minimum screw speed in the application or tightening stage
I _A shutdown motor current at the end of the application or tightening stage

The curve representations according to FIGS. 5 and 6 also apply in this case, the motor current instead of the screwing torque being to be indicated only on the ordinate of FIG.

REFERENCES

• 1 screw spindle
• 2 mounting plate
• 3 workpiece
• 4 length meters
• 5 reduction gears
• 6 Motor output shaft
• 7 torque sensor
• 8 flange
• 9 bearing housing
• N A drive shaft 11 of the screw
• 11.1 End of delivery
• 13 A b drive shaft of the screw
• 13.1 End of recording
• 15 square
• 17 protective sleeve
• 18 threads
• 20 spool
• 22 coil spring
• 24 depth sensor
• 26 bridge
• 28a-d strain gauges
• 30 - 33 points of the bridge
• 35 memory chip
• 37 Electronic switch (NPN transistor)
• 38 resistance
• 40 exit
• 42 clock input

• A (index) A bschaltgröße
• I Actual motor current
• An L egestufe l
• M tightening torque
• n screw speed
• t time
• V dwell time
• Z At such iehstufe

Claims (7)

1. Screw spindle (1) with
a) a drive shaft (11) and an output shaft (13) axially displaceable against it against the force of a spring,
characterized by
b) a depth sensor (24) for measuring the screw-in depth, which is formed in the following way:
b1) the torque output end (output end 11.1) of the drive shaft (11) is axially displaceably mounted within the torque receiving end (intake end 13.1) of the output shaft (13),
b2) in a housing part (sleeve 17) of the screw spindle, which surrounds at least parts of the delivery end (11.1) and the receiving end (13.1), a coil (20) is accommodated, within which the receiving end (13.1) can be moved while changing the inductance of the coil is.
(Figure 1) 2. Screwing machine with a screw spindle according to claim 1, characterized in that on a mounting plate (2) on which the screw spindle (1) is to be fastened, next to the screw spindle or on two or more sides of the screw spindle one, two or more per se Known length meters (4) are arranged so that they measure the distance (s) between the mounting plate (2) and a workpiece (3) on which the screwing operation is to be carried out, parallel to the depth sensor (24). (Figure 1) 3. screwing with a screw according to claim 1, characterized by the combination with the following additional features:
a) a motor (in particular with a reduction gear (5)),
b) a torque sensor (7) which is arranged between the motor and the drive shaft (11) of the spindle,
c) a control circuit for the screwing process,
d) a speed sensor connected to the control circuit for measuring the motor or spindle speed,
e) an electronic memory module (35), in which parameters of the screw spindle, its drive and the sensors are stored, accommodated within the screwing machine, in particular the screw spindle,
f) the memory module can be connected to the control circuit via the torque sensor (7).
(Figure 2) 4. Screwing machine according to claim 3, characterized by the following features:
a) the torque sensor has two or in particular four strain gauges (28a-d) connected in a bridge,
b) the size of the electrical resistance of one of the branches of the bridge can be changed arbitrarily via an electronic switch (transistor 37), in particular by connecting a fixed resistor (38) in parallel, in such a way that a bridge circuit and the control circuit for transmitting Control signal supplied to the setting data changes,
c) a pulse train is used to actuate the electronic switch, which the memory module emits under the influence of clock pulses supplied by it.
(Figures 2 - 4) 5. The method for operating a screwing machine according to claim 3, wherein the screwing process is carried out in two process stages separated by a dwell time, the application stage and the tightening stage, each of which when a predetermined shut-off screwing torque (M _AL , M _AZ ) or Shutdown motor current (I _AL , I _AZ ) is terminated, characterized in that in the application stage, under the control of the depth sensor, the screw speed n from its initial value when the application state is approached, in which the screw head or the nut touches the base. is reduced to a final value (cut-off screw speed n _AL ), which allows stopping without overloading when the cut-off screw torque (M _AL ) or cut-off motor current (I _AL ) is reached. 6. The method according to claim 5, characterized in that the respective screw speed n is regulated while continuously measuring the respective screw torque M according to the following formula:

in which mean:
n _max maximum screw speed in the application or tightening stage
n _min minimum screw speed in the application or tightening stage
M _A switch-off tightening torque at the end of the application or tightening stage 7. The method according to claim 5, characterized in that the respective screw speed n is regulated while measuring the respective motor current (actual motor current value) I according to the following formula:

in which mean:
n _max maximum screw speed in the application or tightening stage
n _min minimum screw speed in the application or tightening stage
I _A shutdown motor current at the end of the application or tightening stage

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